1. Field of the Invention
The present invention generally relates to phase noise reduction in oscillator circuits, and more specifically to phase noise reduction in differential crystal oscillator circuits.
2. Background Art
Radio frequency (RF) transmitters and receivers perform frequency translation by mixing an input signal with a local oscillator (LO) signal.
Preferably, the LO signal should have a frequency spectrum that is as close to a pure tone as possible in order to maximize system performance during the signal mixing operation. The deviation of the LO signal from a pure tone is quantified as phase noise or phase jitter, and is generally referred to as spectral purity. In other words, a LO signal with good spectral purity has low phase noise.
Typically, the LO signal is generated from a lower frequency reference signal in order to maximize spectral purity. The lower frequency reference signal is often frequency multiplied to generate the higher frequency LO signal. For instance, a phase lock loop (PLL) generates an output signal that is a frequency multiple of an input reference signal, but is phase-locked to the input reference signal. In some applications, several multiplication stages are required to achieve the desired LO frequency.
Frequency multiplication can negatively impact spectral purity by increasing phase noise in the output LO signal. Phase noise increases because frequency multiplication (which is equivalently phase multiplication) enhances phase noise spectral density as the square of the multiplication factor. Therefore, the higher order multiplication of a noisy reference signal is to be avoided.
A crystal oscillator is often used to generate the reference signal because of its inherently low phase noise attributes. A crystal oscillator includes an active device and a crystal, where the impedance of the crystal is a short (or an open) circuit at a natural resonant frequency. By connecting the crystal in parallel with the active device, a positive feedback path is created between the oscillator terminals at the crystal resonant frequency. The positive feedback causes the active device to oscillate at the crystal resonant frequency.
A crystal resonator has a relatively high quality factor, or “Q”, when compared to other types of resonators. Therefore, the bandwidth of the crystal resonance is relatively narrow so that the impedance change of the crystal in the vicinity of its resonant frequency is relatively abrupt. The relatively high Q of the crystal improves the spectral purity of a crystal oscillator output signal because the crystal resonance determines the frequency of oscillation for the active device in the oscillator. Accordingly, a crystal oscillator has a relatively low phase noise compared to other resonant oscillator configurations.
The active device in the crystal oscillator typically includes one or more transistors that can be configured in various arrangements. Transistors necessarily require some type of bias circuitry to power the transistors. The bias circuitry typically includes one or more resistors, which inherently produce thermal noise that is proportional to the total resistance. The thermal noise voltage modulates the zero crossings of the oscillation waveform, and increases the phase noise floor around the oscillation frequency. The increased phase noise floor detracts from the inherently low phase noise of a crystal oscillator. Additionally, as stated above, a high phase noise floor is undesirable in reference signals that drive frequency synthesizers because the output phase noise increases with square of any frequency multiplication that is performed by the synthesizer.
Therefore, what is needed is an oscillator circuit architecture that nullifies the thermal noise voltage that is created by the bias resistors that power the active device in the oscillator circuit.